Due to the high impedance inputs of all modern guitar amplifiers (anything built after 1980), you can simply connect a line level output of a mixing console to the instrument level input of your amplifier to achieve the desired result (There will be no impedance mis-match, as opamp inputs have >1M input impedance).
This circuit adds a few features that my be useful; The transformer isolates ground of the mixing console and guitar amplifier, and the attenuation pot is purely for convenience.